1. Field of the Invention
The present invention relates to a sheet processing apparatus and, more particularly, to a sheet processing apparatus having a sheet buffer function.
2. Description of the Related Art
In recent years, in some image forming apparatus, a sheet processing apparatus is set adjacent to a paper discharge unit of an image forming apparatus main body in order to perform finishing for sheets after image formation such as sorting of the sheets or stapling the sheets.
The sheet processing apparatus has plural means for conveying a sheet, which is conveyed from the image forming apparatus, to a paper discharge tray and discharging the sheet. The means are roughly divided into a conveying path for not performing the finishing and a conveying path for performing the finishing. When the finishing is not performed, the sheet is conveyed through the conveying path for not performing the finishing and directly discharged onto the paper discharge tray. When the finishing is performed, the sheet is conveyed to a processing tray through the conveying path for performing the finishing, which is branched from the conveying path for not performing the finishing, and stacked. When a set number of sheets are stacked, the sheets are aligned on the processing tray and subjected to the finishing.
Conventionally, there is known a sheet processing apparatus that has first and second conveying paths provided in a processing conveying path further on an upstream side than a processing unit and includes superimposing means for conveying a sheet through the first and second conveying paths, temporarily stopping the sheet, and superimposing plural sheets one on top of another, wherein the superimposing means shifts the superimposed sheets on an upper side to slightly precede the sheets on a lower side (e.g., JP-A-11-157741).
U.S. Pat. No. 7,172,187 discloses a sheet processing apparatus that puts, when finishing is necessary, plural sheets on standby on a waiting tray, causes the sheets to fall onto a processing tray provided below the waiting tray with own weight of the sheets, stacks a predetermined number of sheets, and, after aligning the sheets, staples the sheets.